1. Field of the Invention
The invention generally relates to motor control. In particular, the invention relates to brushless direct current (DC) motors with electronic commutation.
2. Description of the Related Art
A stepper motor is a type of electric motor that xe2x80x9cstepsxe2x80x9d or rotates in an increment in response to an energizing of the stepper motor""s windings. To rotate a stepper motor, the windings of the stepper motor are activated or energized in a sequence. In addition, stepper motors can generate a xe2x80x9cholding torquexe2x80x9d to maintain a particular position. These features make stepper motors quite useful in many automated control applications.
A stepper motor is controlled by a translator or sequencer circuit, which determines the sequence by which the windings of the stepper motor are energized for a particular desired motion. A driver circuit further buffers the control signals from the translator circuit and switches current to the windings of the stepper motor.
Conventional stepper motor drive circuits are inefficient. For example, where the stepper motor drive circuit applies a constant current at a relatively high voltage to windings of the stepper motor, the efficiency of the combination of the motor and the drive circuit can be as low as 10 percent. This low efficiency of prior drive circuits has limited the applicability of stepper motors to those applications where low efficiency can be tolerated. However, many mechanical systems, including those that are powered by batteries, remain relatively sensitive to inefficiency.
U.S. Pat. No. 4,136,308 to Kenyon M. King, entitled xe2x80x9cStepping Motor Control,xe2x80x9d discloses a control circuit for operating a stepper motor from a DC power source by switching current successively through four stator windings. U.S. Pat. No. 4,136,308 is incorporated by reference in its entirety. The switching time, as related to shaft position, is controlled by sensing the back electromotive force (EMF) in the stator coils of the stepper motor. The voltages across two windings of the stator at the transformer are used to generate a commutation pulse each time the EMFs from the stator windings are equal. The commutation pulses are then used to generate clock pulses for sequencing the stator windings. Because the phases of the EMF across two windings are used to generate the commutation signal, the commutation signal is virtually independent of sequence rate.
Embodiments of the present invention are directed to apparatus and methods that control motor power in an electric motor, such as a stepper motor. Embodiments of the invention advantageously allow improved open-loop motor performance, allow reduced motor hunting, detect stalls, detect out-of-synchronization errors, control and synchronize multiple motors, and can automatically configure the starting and stopping sequence times, for a given load, to optimize performance. One embodiment is an EMF commutation circuit that detects a rotor position at which to sequence the motor to provide an optimized performance from the motor. Another embodiment includes pulse width modulation (PWM) to control power, and the embodiment measures the rotor position within a detent position. Embodiments of the invention can utilize a motor controller adapted to respond to a commutation signal, a commutation pulse, or both in conjunction with a sensing device, such as an EMF detector or an optical encoder.
Embodiments of the present invention overcome the disadvantages of the low-efficiency of conventional motor drive processes by using a commutation signal from external rotary sensors or a commutation signal derived from the EMF to develop relatively efficient power control. The relatively efficient power control supplies a motor with enough power for the motor to drive its load and preferably, nothing more. The improved efficiency power control can also be combined with anti-hunting logic, which reduces the hunting of the motor by monitoring the rate of the commutation signal with respect to the sequencing. In another embodiment, an improved EMF commutation process develops a commutation signal at a relatively high sequence rate and a detent position sensor PWM commutation at a relatively low or stopped sequence rate. This enables the detection of a traversal of a detent position upon stopping, and allows an embodiment according to the present invention to ascertain the fastest deceleration slope it can use without inducing errors such as traversals. Further, embodiments of the present invention include routines that dampen oscillations after a stop. The use of the EMF commutation signal is also instrumental in the control and synchronization of multiple motors.
The techniques disclosed herein apply generally to a wide variety of electric motors including direct current (DC) motors, alternating current (AC) motors, stepper motors, variable reluctance (VR) motors, permanent magnet (PM) motors, multiple-phase motors (such as 2 or 3 phase motors), linear motors, and the like. It will be understood by one of ordinary skill in the art that an applicable motor should be appropriately wound or have a stator/rotor configuration that allows independent control of each motor winding.
The invention overcomes the disadvantages of the prior art by improving the efficiency of electric motors with commutation control. One embodiment of the invention uses an external rotor sensor or uses the EMF to provide a commutation signal, which controls the power applied to the motor such that the motor is driven with relatively little excess drive and at a relatively high efficiency.
One embodiment of the invention further includes advanced anti-hunting logic that advantageously reduces the hunting of the motor by monitoring the rate of change of the commutation signal with respect to the sequencing of the windings.
One embodiment of the invention is a technique to control a motor so that it stops relatively quickly. At relatively low sequence rates including zero, a detent position sensor (PWM commutation) is used to develop a commutation signal. At relatively high sequence rates, an improved EMF commutation process develops the commutation signal. Advantageously, embodiments of the invention can decelerate at relatively high speeds and can stop with a relatively high amount of damping so that the motor stops with relatively little oscillation or resonance. The improved EMF commutation process can also advantageously be used to control and to synchronize the motors of a system with multiple motors.
One embodiment of the invention includes an improved EMF detection technique that develops the EMF commutation signals when EMF is present. When the EMF is absent or low, a PWM commutation signal represents the imbalance in the charging currents of the two motors windings that are on. The imbalance represents the offset of the rotor from the rotor""s zero detent position.
One embodiment includes an EMF commutation sensing process and a PWM commutation sensing process for bipolar wound motors.
Another embodiment includes a process that controls motor power based on the timing from sequencing to the arrival of the commutation signal.
Another embodiment includes a process that reduces motor hunting based on the rate of change of the commutation signal with respect to the sequencing of the motor.
Another embodiment includes a process that improves open loop performance using the commutation signals to control power, hunting and instantaneous sequence time.
Another embodiment includes a process that determines if a detent position has been traversed, using the improved PWM commutation detection technique.
Another embodiment includes a process that derives signals to actively dampen and to position a rotor within a detent position using the improved PWM commutation detection technique.
Another embodiment includes a process that automatically determines the motor parameters that provide each power/load/motor combination with the best performance.
Another embodiment includes a process that detects stalls, motor torque and out-of-synchronization errors and a corrects the detected errors.
Another embodiment according to the present invention includes a technique for controlling and synchronizing multiple motors.